As a method for manufacturing an optical fiber preform, a rod-in collapse method is well-known. This rod-in collapse method involves forming a glass having at least a core portion into a rod shape, forming the glass for a cladding into a glass pipe having a great thickness, inserting the rod into the glass pipe, and melting the rod and the glass pipe in a state where the pressure within the glass pipe is set to be lower than outside the glass pipe, while heating, to produce an optical fiber preform having the core and the cladding. Using a produced collapsed body as a preform intermediate product, a cladding portion may be composed around its outer circumferential portion to make a large preform by vapor phase deposition such as VAD method and OVD method, or a rod-in collapse method.
With this method, dummy pipes 3a, 3b are connected with a glass pipe 2 for forming the cladding, as shown in FIG. 15A. The glass pipe 2 is set near a not-shown heat source of a resistance furnace, a high frequency furnace, or an oxyhydrogen flame, etc., for performing the collapse so that the central axis may be vertical (vertical type). An inner surface of the glass pipe 2 is etched to smooth it and remove impurities. Then, a glass rod 1 having at least a core portion (hereinafter simply abbreviated as a glass rod) is pushed up by a dummy rod 4 and inserted into the glass pipe 2. The glass pipe 2 with the glass rod 1 inserted is baked in a chlorine gas atmosphere and dried, and the impurities are removed. Then, the glass pipe 2 is collapsed from an upper or lower portion to produce the optical fiber preform. With this method, the glass pipe 2 may be possibly collapsed in a state where the glass rod 1 is inclined, as shown in FIG. 15A. A portion softened by heating is deformed under the gravity, as shown in FIG. 15B, whereby there is a problem of causing the core of the produced optical fiber preform to be eccentric and deformed.
When the glass rod 1 is placed in a horizontal direction (horizontal type) as shown in FIG. 16A, the dummy pipes 3a, 3b are connected to both ends of the glass pipe 2 by the conventional method, a reduced diameter portion 5a, 5b being formed in a part of the dummy pipe 3a, 3b. Then, the glass rod 1 is inserted into the glass pipe 2 and the dummy pipes 3a, 3b as shown in figure, fused and fixed with the reduced diameter portion 5b of one dummy pipe 3b, and collapsed from the side of the dummy pipe 3a that is opposite to the fixed side. In this case, the heating amount is increased, especially when the outer diameter of the glass pipe 2 is 45 mmφ or more. If the glass rod 1 is softened by heating, the glass rod 1 is moved so that a heated portion is deformed, as shown in FIG. 16B. Therefore, the produced optical fiber preform has the same problem as with the vertical type.
In recent years, a dispersion compensating fiber has been developed for compensating a chromatic dispersion that occurs in making the optical communication in a wavelength band of 1.55 μm, employing one of the optical fibers having a zero dispersion in a wavelength band of 1.3 μm. Since a zero dispersion fiber for 1.3 μm wavelength band causes a great positive chromatic dispersion in a wavelength band of 1.55 μm, the dispersion compensating fiber is required to have a great negative chromatic dispersion that is inverse to the positive chromatic dispersion in the wavelength band of 1.55 μm to compensate this chromatic dispersion.
Therefore, the dispersion compensating fiber has a structure that a relative refractive index difference Δ between the core and the cladding is increased (usually about 0.35% for a single mode fiber most typical for 1.3 μm transmission and about 1.0 to 3.0% for the dispersion compensating fiber) by the addition of dopant, and the core diameter is reduced (usually about 8 to 10 μm for the single mode fiber and about 2 to 6 μm for the dispersion compensating fiber).
The dispersion compensating fiber is likely to cause a polarization mode dispersion (PMD) because the core of high refractive index is employed, and is likely to be elliptical because the glass of the core portion has a lower viscosity due to influence of GeO2 doped into the core. A non-circularity of the core is represented by the numerical formula 1, when regarding the core as substantially elliptical, in which the core is more round for the smaller non-circularity.
(Numerical Formula 1)
Non-circularity=(length of major axis−length of minor axis)/length of major axis×100(%)
PMD is increased in proportion to the non-circularity of the core. It is known that especially when the relative refractive index difference Δ is higher, the non-circularity of the core has more effect on degradation of PMD. Accordingly, the dispersion compensating fiber with high relative refractive index difference Δ is required to have a small non-circularity.
The dispersion compensating fiber is required, for example, to have the polarization mode dispersion characteristics excellent for a WDM transmission system (system for transmitting a large amount of information corresponding to multiple times the conventional information by inputting a plurality of signal having different wavelengths) having 10 Gb/s (gigabits/second) per wave or more, and to prevent the non-circularity of the core.
However, in the dispersion compensating fiber of the described structure, the glass rod containing a large amount of dopant is likely to be deformed due to heating, when the rod-in collapse is made. If the thickness of the glass pipe is increased to produce a large optical fiber preform, a large amount of heat is needed in a heating/integrating process by the conventional collapse method, deforming the core into elliptical shape to make the non-circularity worse, resulting in a problem that it is difficult to obtain the excellent polarization mode dispersion characteristics. When the glass pipe of large diameter was employed to produce the large preform by the rod-in collapse method, this problem was more remarkable.
In view of the above-mentioned problems, it is an object of the present invention to provide a method for manufacturing an optical fiber that is rounder than by the conventional method, by reducing the non-circularity of core with the rod-in collapse method, an optical fiber preform with a reduced non-circularity, and an optical fiber.